By Deepu Dharmarajan
Posted 23 days Ago

SIGNALLING A LAYOUT | PART 3

Signalling

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CONTINUED FROM - SIGNALLING BOOK | CHAPTER 3 | PART 2

SIGNALLING BOOK | CHAPTER 3 | PART 3

CONTENTS

1. Introduction - In Part 1

2. Headway - In Part 1

3. Positioning of Running Signals - In Part 2

4. Types of Signal - In Part 2

5. Points and Crossings - In Part 3

6. Track Circuits - In Part 3

7. Identification of Signals, Points & Track Circuits - In Part 3

8. Examples - In Part 3

 

5. POINTS AND CROSSINGS

Although, in general, the siting of points and crossings on an existing railway will be dictated by permanent way design considerations, it is left to the Signal Engineer to determine the operation of the points. Furthermore, the Signal Engineer may require additional trapping protection to be provided on occasions and such cases must be referred back to the Permanent Way Engineer or other responsible engineer.

In the case of combined track remodelling and resignalling projects, it is sometimes possible to provide simpler or improved signalling controls by minor alterations to the track layout. Close co-operation between the Signal Engineer and the Permanent Way Engineer is essential if the optimum results are to be achieved.

5.1. Position and Numbering of Points

Any set of points will be defined as lying in its Normal position for one route and its Reverse position for the other route. The Normal position of the points will be shown on the signalling plan as follows:-

A similar convention applies to switched diamond crossings, if used.

Points should be numbered in such a way that any point ends required to work simultaneously carry the same number. To localise failures, it is not advisable to number more than two ends to work together. In addition, Solid State Interlocking (SSI) equipment is normally only configured to operate single and double ended points, although in certain circumstances three ends can be accommodated.

For control purposes, each end has to be identified separately (A or B) but this may not need to be shown on the signalling plan. A convention must be determined for identifying A and B ends (e.g. A end nearest control centre or A end at lowest reference distance etc.) and strictly observed. On TfNSW network  a down train will meet the A end first.

5.2 Ground Frames

Ground frames control infrequently used points, usually outside interlocking areas. Although referred to as ground frames, they may equally well be locally operated control panels.

In its most common form the ground frame consists of just 2 levers, the point lever and a release lever (which will also work the F.P.L. if the points are normally facing). Movements over the points during shunting are usually controlled by handsignal, although extra levers may be provided to control or slot signals which the train must pass during shunting. Note the use of separate releases where the ground frame controls more than one function.

Instead of providing an electric lock on the release lever, a separate key is electrically released when the signalman operates the release button. This key is then used to release the ground frame release lever. It remains captive until the ground frame is normalised and can then be returned to the instrument to give back the release.

5.3 Trapping Protection

It may be necessary to request trap points (normally known as catch points on TfNSW) to be provided at certain locations:

  1. At the exit from sidings, where they lead on to running lines, catch or trap points must be provided to prevent an unattended vehicle running away or a shunting movement overrunning and fouling the running line.
  2. Where a full overlap cannot be obtained and movements are required to closely approach a converging junction, catch or trap points leading away from the running line can be used as an overrun in place of the normal overlap.
  3. On railways where a distinction is made between passenger and non-passenger lines, trap points may be used where the non-passenger line joins the passenger line.

Where trap/catch points occur in track circuited lines many railways employ a track circuit interrupter to ensure a derailed vehicle which is still fouling the track, although not standing on the rails, remains detected. The track circuit interrupter is normally insulated from the rail on which it is mounted and bonded in series with the opposite rail.

6. TRACK CIRCUITS

Track circuits shall be provided in a manner which permits maximum flexibility with minimum expense and complexity.

6.1 Overlaps

Running signals should, in general, be provided with separate berth and overlap track circuits, the berth track circuit terminating immediately beyond each signal. This will ensure the signal is replaced to danger at the earliest opportunity after the train passes. Where more than one overlap is required, a joint must be provided at the end of each overlap.

Calculation of overlaps has already been covered in the earlier sections dealing with the positioning of signals and trainstops. However, old TfNSW  practice on overlaps is summarised below.

6.1.1. Where Trainstops Are Not Fitted

The overlap is a margin to allow for braking errors. There is no positive means of stopping the train if a driver completely misses or misreads a signal. As such it is an approximate distance based on experience, rather than one which has been calculated on any scientific principle. The standard SRA overlap is 500 metres. This may be smaller or greater than the actual braking distance. Where speeds are low, this is sometimes reduced. Recommended overlap lengths are:-

6.1.2 Sydney Metropolitan Area (Open Sections)

If  trainstops are fitted,(recently ATP rollout has taken place which will remove the significance of  mechanical trainstop) the overlap must be based on emergency braking distance for the prevailing speeds and gradients.

The following example shows how this may be calculated. To simplify the calculation when dealing with gradients it is often easier to express the braking rate as a percentage of the acceleration due to gravity (g = 9.8m/s2).

Braking distance = v2/2a (where a = braking rate, v = train speed)

= 100v2/2g(%B + %G)

%B = Braking rate as a percentage of g

%G = Gradient as a percentage (down gradients negative)

If the line speed is 25 m/s, the braking rate is 10% and the gradient is 1% down, the emergency braking distance and hence the overlap will be:-

100 x 252 / 2 x 9.8 x (10 - 1) = 62500 / 176.4

This would give a minimum overlap of 354 metres. This would probably have to be increased by a suitable margin to allow for less than 100% braking performance (e.g. some brakes isolated, wet or greasy rails, delay time for brake application).

Where available, braking tables or curves should be used.

If the full overlap is foul of junctions or station platforms, a reduced overlap should be considered. The train speed would have to be suitably reduced by a low speed or conditional caution aspect at the previous signal.

6.2 For Points and Crossings

The positioning of track circuit joints to prove clearance will depend on the dimensions of the rolling stock in use. One must first determine the difference between the maximum vehicle width and the width of the widest vehicle in service. The position must then be found where the rails leading away from the crossing are at least this distance apart (normally adding a small safety margin). At this point, the extreme ends of vehicles on the adjacent tracks will not be foul of each other.

Measuring away from this position, the joint must be located at a distance greater than the maximum end overhang of any vehicle. This is obtained by measuring from the centre of the outer axle to the extreme end of the vehicle.

Track circuits should allow maximum flexibility of use of the layout. In particular, where the track layout permits parallel moves, the signalling must not prevent them. Joints should be positioned to achieve the earliest release of points after the passage of a train consistent with safety, economy and practicality of installation.

Example 1

Joint A allows simultaneous moves over both ends of crossover normal.

Joint B allows points to be moved as soon as train leaves points.

Joints C, at clearance point, allow movements across crossover with Tracks X and Y occupied. It is common on plans to place joints C opposite the tips of the points.

Example 2

Joints A allows parallel moves.

Joints B allow points to be freed as soon as junction cleared.

Joints C are set back at clearance point. These may also be the overlap joints for signals approaching the junction.

Joint D will be dependent on factors other than the requirements for operation of the junction, eg. the position of the protecting signal.

7. NUMBERING OF SIGNALS, POINTS AND TRACK CIRCUITS

To enable all signalling controls to be specified, each signalling function must be uniquely identified. It aids design, testing and fault location if this is done in a logical and orderly manner. In particular, confusion is avoided if different types of functions are numbered in different number or letter series.

The main functions which need to be numbered are:-

  1. Main Signals
  2. Shunt Signals
  3. Points
  4. Track Circuits
  5. Ground Frame & other releases

Separate number series should be provided for each type of function (points, signals etc.). Main and shunt signals may be numbered in the same or separate series.

Lines for each direction of traffic are normally designated UP and DOWN. Signals reading in the Down direction normally carry odd numbers with the lowest number at the Up end of the control area. Signals reading in the Up direction normally carry even numbers, again with the lowest number at the Up end.

Points will be numbered with the lowest number at the Up end of the control area.

Where possible suitable gaps should be left in the numbering sequences in anticipation of future alteration. Distinct branches should be numbered in separate series.

Historically, several different conventions have been used for identification of track circuits. Each has advantages and disadvantages. One common method is to use a simple numbering sequence. The disadvantage of numbers is that, on a large installation, very large numbers or duplicate number sequences need to be used (with greater risk of errors in design and testing). Another alternative which has been used is to number track circuits based on the distance along the line. This results in track circuits in one locality having long and very similar numbers. Again confusion and errors may result.

TfNSW  uses a system based on the signal numbers. The first track past signal 5 would be 5A, the next 5B and so on. The suffix E is not normally used.

The BR standard is now to use letters. Each track circuit indicated to the signalman should be identified using two capital letters, arranged alphabetically in a logical sequence. Letters I and O are not used. Where a number of track circuit sections have a common indication they should have the same identity plus an individual suffix number, eg. AA1, AA2 etc. This arrangement is simple but does not give any indication of the relative locations of tracks and signals.

8. EXAMPLES

A few examples are now given of some of the more commonly found track layouts and suggested arrangement of signals. They do not cover all situations. In practice, different requirements will conflict. The signal engineer must resolve these conflicts in the most effective and economic manner

8.1 Junctions

The logical arrangement at a junction is for the protecting signal to be as close to the junction as possible. For diverging movements this ensures that trains are not checked too far from the junction, while for converging movements it reduces the chances of trains being checked due to conflicting moves on the junction.

The signal next in rear of the junction cannot be cleared unless the section is clear up to the junction signal, and the overlap beyond. It is preferable that the overlap is not fouled by conflicting moves, so ideally the signals protecting the junction should be placed overlap distance in rear of the junction. If large overlap distances make this impractical, a reduced overlap clear of the junction should be considered. In this case the signal in rear should have its full overlap clear of the junction.

8.2 Station Platform with Loop

It is usually desirable for headway reasons to site a signal at the end of the platform. This provides a platform starting signal and also protection for any level crossing at this point. In situations where there is no platform starting signal, there is a risk that a station stop will divert the driver's attention sufficiently for him to forget the aspect displayed by the previous signal. After restarting his train, he could approach the next signal at an unsafe speed.

In the above example, signal A is located at full S.B.D. from signals B & C. For main line running this is satisfactory, but for a move into the loop the time taken for the train to slow to 25 km/h over the crossover may adversely affect the headways. A better arrangement is shown below :

Signal A is moved closer to the turnout. If this results in inadequate braking distance from signal A to signals B & C, signal D must be a 4 Aspect signal. Signal A would only need to be 4 aspect if the signal ahead of C was less than braking distance.

8.3 Terminal Stations

The station throat track capacity must be at least double that of the approaching line. This
is because some arriving and departing movements will completely block all other routes.

The signal reading into the platforms should be as close as possible to the plaforms. If this signal does not have an overlap clear of points, the signal in rear should do so to allow trains to approach unrestricted.

Signal spacing on the approach to the station should be as close as possible consistent with standing room and headway requirements.

The first signal leaving the station should be as close as possible (whilst retaining necessary train standage clear of the pointwork), to allow the best possible aspect on the platform starting signals. The standage requirement may have to be reduced to maintain adequate line capacity on the departing line.

The platform entry signal 3 exhibits stop and caution aspects only for running moves. Buffer stops are equivalent to a signal permanently at stop.

Subsidiary shunt signals are provided to enable trains or locomotives to enter occupied platforms.

If wrong line shunting is required, a shunt limit board must be provided at a position which permits adequate standing.

 

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Deepu Dharmarajan - Posted 258 days Ago

THE PURPOSE OF SIGNALLING

SIGNALLING BOOK | CHAPTER 1 CONTENTS 1.Introduction 2.The Problems to be Solved 3.Basic Requirements 4.Lineside Signals 5.The Absolute Block System 6.Interlocking of Points and Signals 7.Single Lines 8.Further Developments 1. INTRODUCTION In general, the railway traveller assumes  that  his  journey  will  be safe. This  high  standard of safety which is taken for granted is the result  of  a  long  history  of  development.  As human errors and deficiencies in safety systems become evident, often as a result  of  an accident, improvements are made which are then incorporated into new generations of equipment. This is certainly true of railway signalling. It also appears to  be  a continuing  process.  We have not yet reached the situation where absolute safety can be assured. It is useful to start by looking back at some of the early history of signalling development. In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more  powerful  locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of  control over train  movements  became  necessary.  The need  for railway signalling had been identified. 2. THE PROBLEMS TO BE SOLVED 2.1 Collision with a Preceding Train When one train follows another on to the same section  of  line,  there  is a  risk  that,  if  the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were separated using  a system of  "time  interval"  working,  only  permitting a train to leave a station when a prescribed time had elapsed after the departure of  the previous train. Although this reduced the risk  of  collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence  of  any proper communication between stations, it was the best that could be achieved at that time. 2.2 Conflicting Movements at Junctions Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some  method  of  regulating  the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other. 2.3 Ensuring that the Correct Route is Set Where facing points are provided to allow a train to take alternative routes, the points must be held in the required position before the train is allowed to proceed and must not be moved until the train has completely passed over the points. Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them. 2.4 Control of Single Lines Where traffic in both directions  must  use  the same single  line,  trains  must  not  be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable,  problems  could  still  arise if  trains were delayed or cancelled. 3. BASIC REQUIREMENTS We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:- To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead. To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly. To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them. To prevent opposing train movements on single lines. All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the  frequency  demanded  by  the  timetable  to meet commercial requirements. This must be done without reduction of safety below an acceptable level. Signalling involves not only the provision of  equipment  but the adoption  of  a consistent  set of operating rules and communication procedures which can be understood and implemented by all staff responsible for railway operation. 4. LINESIDE SIGNALS It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground who has sufficient knowledge of the current traffic situation. His decision must be passed on to the driver of each train passing through his area of control. In the early days railways employed policemen whose duties would include the display of hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties. The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal  on the approach side controlling entry to the station and a "starting" signal  protecting  the section of line to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications,  STOP  or  PROCEED. They therefore became collectively known as "stop" signals. As line speeds increased, "distant" signals were introduced which gave advance warning of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication. With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance. It is important to understand the difference between stop and distant signals. A train must never pass a stop signal at danger. A distant signal at caution can be passed but the driver must control his train ready to stop, if necessary, at a stop signal ahead. The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop. If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indications to act as both stop and distant signals. 5. THE ABSOLUTE BLOCK SYSTEM Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible. On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost. 5.1  Block Sections In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes. The accompanying diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line. Signalbox A controls entry to the block section but it is only signalman B who  can see a train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear. Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section. 5.2 Block Bell The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires). The use of a bell enforces the use of a standard set of codes for the various messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be provided as well, often using  the same circuit as the block bell. 5.3 Block Indicator This provides the signalman at the entrance to the  section with a continuous visual indication of the state of the section, to reinforce the bell codes. It is operated by the signalman at the exit of the block section. Early block instruments were "two position" displaying only two indications;  line clear and line blocked. Later instruments display at least 3 indications. The most usual are:- Line clear Giving permission to the rear signalman to admit a train to the section. Normal or Line Blocked Refusing permission. The signalman at the entrance to the section must maintain his starting signal at danger. Train on Line There is a Train in the block section. 5.4 Method of Working When signalbox A has an UP train approaching to send to box B,  the signalman at A will offer it forward to box B, using the appropriate bell code (so that signalman B knows what type of train it is). If the signalman B is unable to accept the train for any reason, he will ignore A's bell, and leave the UP line block indicator at "Normal". If he is able to accept the train, signalman B will repeat the bell code back to box A, and change the indication to "Line Clear". When signalman A sees  his  block  repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section. When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge this by repeating the bell code back to A, and turning the block indicator to "Train on Line". When the train leaves the block section at B, the signalman  there checks  that it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again.  He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train. On multiple track railways, a pair of block instruments as above is required for each line. 5.5 Extra Safeguards The basic three-position block system, as described, relies on the correct sequence of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure, and the times at which the bell signals are exchanged. In most places, additional safeguards have been added to the basic system. An  electric lock on the starting signal will prevent it being operated unless the block indicator is at line clear. Track circuit occupation may be used to set the block  instruments to ''Train on Line" if the signalman forgets to do so. Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach. Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The  block system, by ensuring that only one train may occupy a section of line at any time, maintains a safe distance between following trains. 6. INTERLOCKING OF POINTS AND SIGNALS On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains. A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers  could be directly interlocked with each other. This had the following benefits:- Signals controlling conflicting routes could not be operated at the same time. A signal could only be operated if all  the points were in the correct position. The points could not be moved while a signal reading over them was cleared. In early signalling installations, all point and signal operation, together with any interlocking, was mechanical. Although it was a great technological advancement to be able to control a station from one place, the effort required to operate the levers restricted control of points to within about 300 metres from the signal box and signals up to about 1500 metres. At large stations, more than one signal box would often be necessary. The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger. 7. SINGLE LINES On most single line railways trains are infrequent. It is not normally necessary for two trains to follow each other closely in the same direction. Single lines were therefore treated in the same way as a normal block section with the important extra condition that trains could not be signalled in both directions at the same time. To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction. If the timetable required two trains to travel over the single line in the same direction, the driver of the first train would be shown the token (or train staff  as  it is commonly  known) to assure the driver that no other train was on the single line. His authority to enter the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system would cause problems if trains did not work strictly to the timetable. A further improvement was to provide several tokens, but to hold them locked in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument. If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier. 8. FURTHER DEVELOPMENTS The main functions of the signalling system had now been defined, although they were to be continuously improved as the available technology developed. All  signalling  systems would be required to maintain a safe distance between trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled. In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account. In many cases the final decision on the type of signalling to be provided is outside the direct control of the signal engineer. However, he should always endeavour to provide the best possible information and propose cost-effective solutions to particular problems so that the best decisions can be made.

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Deepu Dharmarajan - Posted 258 days Ago

BASIC SIGNALLING PRINCIPLES | PART 1

SIGNALLING BOOK | CHAPTER 2 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Signal Aspects - In Part 1 3. Signalling Principles - In Part 2 4. Drawing Standards - In Part 2 5. Interlocking Principles - In Part 2 6. Train Detection & Track Circuit Block - In Part 2 7. Colour Light Signals - In Part 2 8. Control Panels & Other Methods of Operation - In Part 2 9. Colour Light Signalling Controls - In Part 2   1. INTRODUCTION Whatever type of signalling system is provided on a railway, its basic functions will remain the same. Safety must be ensured by preventing trains colliding with each other and locking points over which the train is to pass. The means of achieving these functions may vary from one railway administration to another but a set of rules must be laid down to define:- The positioning of signals The types of signals The aspects to be displayed by the signals and the instructions to be conveyed  by those aspects The controls to be applied to the signals The method of controlling points The method of interlocking points with signals The standardisation of human interfaces Many countries have sytems of signalling based on British railway signalling practice. The basic British system is very simple having only a small number of different signal aspects displayed to the driver. The driver is responsible for knowing the route over which he is to pass. The signal engineer must, in turn, provide sufficient information for the driver to safely control the speed of his train and, where necessary, to inform him which route he is to take. Other signalling systems have developed along a different path. The driver is given specific instructions to travel up to or reduce to an indicated speed. Route indications are optional. This will generally require a more complex set of signal aspects. This section will deal mainly with the principles and practices of the State Rail Authority of New South Wales, with reference to other systems where appropriate. 2. SIGNAL ASPECTS There are three principal types of signal, each serving a different purpose:- Main or Running signals control the normal movement of passenger and freight trains on running lines. The great majority of movements will be controlled by main signals. Subsidiary signals, mounted on the same post or structure as running signals, control movements other than for normal running, such as the shunting or coupling of trains. Independent shunting signals, generally similar to the subsidiary signals above, are provided for shunting movements at positions where there is no need for a running signal. We will examine the aspects displayed by each type of signal and the instructions and/or information conveyed by them. 2.1 Main or Running Signals As all early signals were semaphore signals, displaying a light for night time use, the aspects of colour light signals are usually based on the indications of the semaphore signals which they replaced. As most new signalling installations are likely to employ colour light signals, this section will concentrate on colour light signalling only. SRA (Currentlly TfNSW) employs two methods of signalling on main lines; single light and double light. As the name suggests, double light signals will always display at least two lights to the driver. Double light signalling is generally used in the Sydney metropolitan area. Single light signals normally use only one light to convey instructions to the driver, although a second marker light may be illuminated to aid the driver in locating the signal. Single light signalling is mostly used on lines outside the Sydney metropolitan area. Although there are similarities between the two systems, we will deal with each separately. We will then make a comparison with the corresponding aspects displayed by the British system to enable readers to read signalling plans drawn in British style. 2.1.1 Double Light Signalling This is intended for use in areas where signals are closely spaced. Each stop signal is therefore required to carry a distant signal for the signal ahead. To give the driver a consistent indications, each signal carries two separate signal heads. The upper signal head can be considered as the stop signal. It will always be capable of displaying, at least, stop and proceed aspects. The lower signal head can be considered as the distant for the next signal ahead. An additional green light may be provided below the distant. This is used for a "low speed" indication . FIGURE 1 shows the normal running aspects for double light signalling. Four aspects are used for normal running :- STOP is denoted by two red lights, one above the other. Note that the lower signal head will always display a red if  the upper signal is at red, even if the signal ahead is showing a proceed aspect. This is important to avoid misleading or confusing the driver. CAUTION is denoted by green over red. In other words, this signal is at "proceed" (top signal head) but the next signal is at "stop" (bottom signal head acts as distant). The caution indication tells the driver to be prepared to stop at the next signal. MEDIUM is a preliminary warning of the need to stop. It is denoted by green over yellow. Signals in urban areas may be closely spaced.  The one  signal  section between the caution and the stop may provide insufficient braking distance for a train travelling at full line speed. The medium indication tells the driver that the next  signal is at caution. This implies that he may have to  stop  at  the  second  signal ahead. CLEAR allows the train to proceed at maximum speed. A clear indication is two green lights. This will tell the driver that there is no need to reduce speed (other than for fixed speed restrictions) before the next signal. All the above indications require the driver to know where the next signal is, to safely control the speed of his train and be able to stop where required. An additional indication is provided on some signal, A  LOW  SPEED  indication, consisting of a small green light below a normal stop aspect tells the driver to proceed at no more than 27km/h towards the next signal. This is generally more  restrictive than the  caution. The low speed aspect is used when the track is only clear for a very short distance beyond the next signal. Fig 1: DOUBLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING *Where a low speed indication is provided. Fig 2: DOUBLE LIGHT SIGNALLING - TURNOUT ASPECTS NOTE: A full clear indication is not given for turnouts. Note the difference in the indications given by Multi-light signals at a turnout. The yellow over red indicates "Proceed" at Medium speed through Turnout, next signal at "Stop". The Yellow over Yellow indicates "Proceed at Medium speed through Turnout" the lower Yellow is cautioning the driver to continue at Medium Speed towards the next signal which is indicating either "CAUTION" or "CLEAR" *Where a low speed indication is provided. **In the Sydney and Strathfield resignaled areas this indication represents a 'low speed' with  the train stop at stop. In this case the signal in the rear will show a caution indication. Figure 2 shows the indications for double light turnout movements. If there is more than one route from a main signal, the driver  must be told whether he is to take the main line or through route or whether he is to take a lower speed diverging turnout. This information is necessary to prevent the driver running through a turnout at too high a speed. The upper signal head is used to display a distinct proceed aspect for a turnout. Instead of the green normally displayed, a yellow light will tell the driver that he is to take the turnout. The proceed aspects for turnouts are:- CAUTION TURNOUT (yellow over red, also described in some operating documents as medium caution) tells the driver to expect the next signal to be at red. MEDIUM TURNOUT (yellow over yellow) tells the driver that the  next  signal ahead is displaying a proceed aspect. This is the least restrictive aspect for a turnout. There is no equivalent of a clear aspect for trains signalled over a turnout. To give a clearer indication to the driver where several routes are possible from one signal, the main signal aspect may be used instead, in  conjunction with a theatre route indicator. The route indicator contains a matrix of small  lunar  white lights which  can  be illuminated to display a Jetter or number. Each character will be associated with a distinct route. The route indicator is not illuminated when the signal is at stop.  When  the  signal  is required. to clear, the route indicator will illuminate for the appropriate route. LOW SPEED aspects may be used. If  a  low  speed  aspect is provided for a turnout route, this is no different in appearance to a  low speed for the main line route. This is not a problem as this aspect conveys a specific speed instruction. As the speed is  likely to be lower than that of most turnouts, route information is not essential 2.1.2 Single Light Signalling On lines where single light signalling is installed, the spacing of signals may vary widely. Therefore some signals may be combined stop and distant signals (as for double  light) but there may also be signals which are stop signals only or distant signals only. The instruction to the driver is therefore generally conveyed by a single light. A second marker light is provided below the main light to aid the driver in locating the signal. The meaning of the signal aspects is equivalent to the double light aspects but the appearance to the driver is different. FIGURE 3 shows the normal running aspects  for single  light signalling. The appearance of each aspect is as follows:- STOP consists of a red light. The marker light also displays a red, except on some older signals where it is lunar white. CAUTION consists of a single steady yellow light. The marker light is extinguished, except on some older signals where it is lunar white. If the main light should fail the marker light  will  display a red on stop signals or yellow on distant signals. MEDIUM, where this aspect is necessary, will be a flashing or pulsating yellow light. The marker light will operate as for the caution aspect. CLEAR is a green light. The marker light will operate as for the caution aspect. LOW SPEED aspects may be used in single light signalling where required. An additional small green light is provided below the marker light. The complete low speed aspect will be a main red light over a red marker light with the additional green light illuminated . Fig 3: SINGLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING   Fig 4: SINGLE LIGHT SIGNALLING - TURNOUT ASPECTS The indication displayed by a Home signal for a turnout movement through facing points into a Loop Refuge siding or important siding consists of a band of three yellow lights in a subsidiary light unit (inclined towards the direction  of the movement). The Red light is  displayed in the Main line signal , as shown. The marker light for the main line signal, contained in the subsiduary light unit will be extinguished when the main line or turnout signal indication is displayed. 4.1 ROUTE INDICATIONS MAIN LINE At locations where more than one turnout is provided one signal indication is some times given and in such cases a route indicator working in conjunction with th e signal is provided, this enables drivers to ascertain the route for which th e signal has been cleared. The route indicator will not show any indication when the signal is at stop, but when the points have been set for the turnout movement a yellow light will appear in the signal in conjunction with the route indication showing a letter to denote the line to which the train will travel, e.g. Figure 4 shows the indications for single light turnout movements. For junction signals, two distinct methods are used according to the situation. For a simple turnout into a loop or siding, a separate turnout signal is provided below the main aspect, incorporating the marker light. For a CAUTION TURNOUT aspect, the main aspect remains at red, the marker light is extinguished and the three yellow lights of the turnout signal are illuminated . The row of lights is inclined in the direction of the turnout. For a MEDIUM TURNOUT aspect, the turnout signal will flash. Otherwise the appearance is the same as above. As for double light signalling, a theatre route indicator may be used in conjunction with the main signal aspect, where several routes are possible from one signal. Again the route indicator is not illuminated when the signal is at stop. When  the signal  is required to clear, the route indicator will illuminate for the appropriate route. The normal construction of signals is to provide a separate lamp unit for each light to be displayed . Some signals, however, are of the "searchlight" type. In this type of signal, the lamp is continuously illuminated and coloured lenses are moved in front of the  lamp according to the aspect to be displayed. The lenses are moved by a relay mechanism inside the signal head. The lights visible to the driver are the same for either type of signal. 2.2 Subsidiary Signals Associated with Main Signals As well as normal running movements, signals may be required for some of  the following movements:- Entering an occupied section Shunting into a siding Running on to a line used for traffic in the opposite direction. Attaching or detaching vehicles or locomotives. The main types are described below. As for the running signals, only current practice is covered in detail. Although the SRA (TfNSW) practice will allow subsidiary signals to clear immediately the route is set, many railway administrations employ approach control to delay clearance of the subsidiary signal until the train has come at or almost to  a  stand  at  the  signal. This  is usually achieved by timed track circuit occupation. The driver will receive a caution at the previous signal and will be preparing to stop. Subsidiary signals are short range signals which are only visible within a short distance of the signal. 2.2.1 Subsidiary Shunt and Calling-on Signals These authorise a driver to pass a main signal at stop for shunting purposes or to enter an occupied section. The driver must be prepared  to  stop short of  any  train or other obstruction on the line ahead. He must therefore control the speed of his train so that he can stop within the distance he can see. The appearance of these signals is a small yellow light below the main aspect. On  some older double light signals the letters "CO" in a round lens illuminated in white may be used. 2.2.2 Shunt Ahead Signal This signal is generally found on single and double lines worked under absolute block conditions. It permits the movement of a train past the starting signal for shunting purposes. It does not require a block release from the signal box ahead and the movement  will  eventually come back behind the starting signal when shunting is complete. As this method of working is generally only found outside the suburban area, a shunt ahead signal will normally be provided on single light signals only. It consists of a small flashing yellow signal below the main running signal and marker light. 2.2.3 Close-up Signal This is similar in appearance and application to the low speed signal. 2.2.4 Dead-end Signal This is for entering short dead end sidings directly from a running line. The only difference between this and a subsidiary shunting or calling-on signal is that the dead end signal is offset from the post on the same side as the siding leads off the main line. Subsidiary signals display no aspect when not in  use.  The  associated  main  signal  will  remain at stop when the subsidiary signal is in use. Route indicators may be used in conjunction with subsidiary aspects to give an  indication to the driver where multiple routes are available. In the case of a movement on to another running line in the wrong direction (i.e. opposite to normal direction of traffic) a route indication is always provided. 2.4 Shunting Signals Signals may also be required for shunting movements in positions where no main signal is necessary. The most common locations are:- Entrance to and exit from sidings. At crossovers to allow a wrong road movement to regain the right line. In yards and depots where main signals are not required. As they have no associated main signal, they must display a stop as well as a proceed aspect. 2.5 Dwarf and Position-light Signals Two main types are in use, the dwarf signal, with all lights arranged vertically and the position light signal. The diagrams show the various signal profiles.  Both types display two red lights for stop. The proceed aspect is normally only a yellow indicating caution. Shunting signals do not always prove track circuits clear and the driver must be ready to stop if there is another train occupying the section ahead. The proceed aspect may be accompanied by a route indication. Shunting signals are normally mounted at ground level, although they may be elevated if required for sighting. 2.3.2 Stop Boards If a wrong road movement is authorised from a shunting or subsidiary signal there must be another signal ahead to limit the wrong road movement. If no such signal was provided, the movement could continue past the protecting signal for the normal direction of traffic and cause a collision. The usual signal is an illuminated notice board carrying the words "SHUNTING  LIMIT". It can be considered as a shunting signal permanently at danger. 2.3.3 Facing Shunt Signals A shunting signal is sometimes needed in a position where it is passed in the normal direction by running movements. To avoid confusing  the driver by displaying a yellow light for a movement which may well be running under the authority of clear signals, these facing shunt signals are provided with an additional green light (clear aspect) for use only in this situation. 2.3.4 Point Indicators Although not signals in the same way as those just described, point indicators are important in sidings to avoid derailments and damage to equipment. They provide a visible indication of the position of hand operated points. Operation may be either mechanical or electrical. Located alongside the point switches, they display an illuminated arrow in the direction of the line for which the points are set. 2.5 British Signalling Aspects For the benefit of those who may at some time have to read signalling plans drawn to British standards (e.g. for the IRSE examination), this is a brief summary of the aspects in use, their meanings and how they are drawn. 2.5.1 Main Signals As with SRA (Currently TfNSW) practice, three colours are used, red,  yellow  and  green.  On  the  plan,  a  red light is denoted by a circle with a horizontal line across it. A yellow  light  has the line at 45° and a green light has a vertical line. The  "normaJ"  aspect  of  the signaJ  (i.e. with  no  routes set and all track circuits clear) is shown by a double line in the appropriate light(s). There are four available aspects; STOP is a red light, CAUTION is a single yellow light, PRELIMINARY CAUTION is two yellow lights and  CLEAR  is  green.The stop, caution and clear signals have the same meanings as the corresponding SRA  aspects. The preliminary caution is similar to the MEDIUM indication  of  SRA  signaJs. The  double yellow aspect is only used in situations where signals  must  be  positioned  closer  together than braking distance. Many lines use red, single yellow and green only. Marker lights are not used. There is no equivalent of the LOW  SPEED  signal. An equivalent control (to allow trains to close up provided they are running  at very low speed)  is provided  by delayed clearance of the yellow aspect. The train must be almost stationary at the signal before the aspect will change from red to yellow. This is achieved by applying approach control with timed track circuit occupation. 2.5.2 Junction Signalling Where a signal has more than one route, a distinct route indication must be given for each  route, except that the highest speed or straight route  need  not have a route  indication.  This may take one of two forms, a junction indicator (a row of  five white lights)  normally  above the main signal and pointing in the direction of the divergence or a multi lamp or fibre optic route indicator displaying one or two characters. There are six available junction indicator positions. Positions 1, 2 and  3 (at 45°, 90" and 135° respectively) indicate diverging routes to the left. Positions 4, 5 and 6 provide equivalent indications to the right. Multi-lamp or fibre optic route indicators are restricted to routes with a speed of 40 mph (64 km/h) or less. Where necessary, clearance of the junction signal is delayed by occupation of the approach track circuit (timed if necessary) to enforce a speed  reduction.  This  is  because  the  driver may receive no warning at previous signals of the route set from the junction signal 2.5.3 Subsidiary Signals The standard subsidiary signal is a position light with two white lights at 45°. The proceed aspect is both lights illuminated. There is no stop aspect -  the  associated  main  signal remains at red. Route indicators are provided  where  necessary but are not obligatory -  if they are provided, route  indications  must  be displayed  for  all routes. The subsidiary signal is used for all shunting and calling on moves. This is a short range signal. An approaching train must be  brought  to a stand  before clearance of the subsidiary aspect. 2.5.4 Shunting Signals The position light shunting signal has two white lights and one red  light. The proceed  aspect is identical to the subsidiary signal. The stop aspect is one red and one white  light, horizontally placed. The white light at the lower right {the "pivot" light) therefore remains continuously lit. A shunting signal with two red lights only is used as a "limit of shunt" indicator. 2.6    Summary Whatever the system of signalling, the signal engineer  must  have  a  detailed  knowledge  of the aspects displayed to the driver and the instructions conveyed. He must then design  the layout of the signalling and the associated controls so that the driver can safely obey all signal aspects. This  must apply for all types of  train likely to use a line. When  required  to reduce  speed or stop, trains must have adequate braking distance under all conditions. The driver of a train must never be given an instruction by a signal that he is unable to comply with.   TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 2 | PART 2...........

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Deepu Dharmarajan - Posted 257 days Ago

SIGNALLING A LAYOUT | PART 1

SIGNALLING BOOK | CHAPTER 3 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 1. INTRODUCTION One of the first steps in any signalling project is to determine the method of train working. Having decided this, it is then necessary to decide the position and spacing of signals. This section will assume throughout that colour light signalling to track circuit block principles will be provided on all main lines. Although other methods of working may well be more appropriate, particularly for lightly used single lines, these will be covered later in the course. It is useful at an early stage to determine whether 2, 3 or 4 aspect signalling will be required. This will be governed by the required line capacity, which in turn will be determined by the timetable to be operated. Having this information and an approximate signal spacing, we can then proceed to position the signals on a scale plan of the track layout. Their position relative to stations, junctions etc. will be decided largely by operating requirements. The most economical arrangement that meets all operating requirements is the one that should be adopted. In order to produce a safe and economical signalling scheme, the designer must use his knowledge of signalling principles and be provided with all necessary details of the train service pattern required, the track layout, gradient profiles, line speeds and train characteristics. If this information is not immediately available, it must be requested from the appropriate authority. Sometimes operating requirements conflict with each other and with safety standards — the engineer must then use his experience to reach a satisfactory compromise whilst maintaining the safety standard. 2. HEADWAY The headway of a line is the closest spacing between two following trains, so that the second train can safely maintain the same speeds as the first. This usually means that the second train is sufficiently far behind the first that its driver does not see an unduly restrictive signal aspect. Headways can be expressed in terms of distance but more usefully as a time (e.g. 2 1/2 minutes between following non-stop trains). It can also be converted to a line capacity (trains per hour). Care must be taken when using a "trains per hour" figure if the trains are not evenly spaced in the timetable. The signalling must be able to handle the minimum headway, not the average. Headway will depend on a number of factors:- D = Service Braking Distance d = Distance between STOP signals S = Sighting Distance (usually 200 yds/metres or distance travelled in 10 seconds) O = Overlap Length L = Train Length (less than 100 yds/metres for a short suburban train but possibly over 1km for a heavy freight train) V = Line Speed (or actual train speed if lower) a = Braking rate Where any of these factors are not given to you, you should always state your assumptions. In practical situations, it is vital to obtain accurate information regarding the braking performance of trains. It is also vital to standardise your units of distance and time. If you work in imperial, yards and seconds are most useful; in metric, metres and seconds would be most appropriate. Whichever you decide, you must use the same set of units consistently throughout to avoid confusion and error. 2.1. Service Braking Distance This is the distance in which a train can stop without causing undue passenger discomfort. It will depend on the line speed, gradient, and type of train. It is usually significantly greater than the emergency braking distance. Theoretically, the Service Braking Distance can be calculated using the line speed and braking rate         This is derived from the 3rd Law of Motion. This calculation will depend upon the braking characteristics of the type(s) of train using the line and must take into account the worst case combination of train speed and braking rate. If this calculation is to be performed frequently, it is useful to show the service braking distances for different combinations of speed and gradient in tabular or graphical form. Gradient should always be taken into account. A falling gradient will increase braking distance, a rising gradient will reduce it. As gradients are rarely uniform between signals, we need to calculate an average gradient using the formula: where G is the average gradient   D is the total distance    g and d are the individual gradients & distances. For a gradient of 1 in 100, G = 100. If the gradient is expressed as a percentage, G is the reciprocal of the percentage gradient. Falling gradients taken as negative, rising gradients as positive. 2.2.  2 Aspect Signalling 2 aspect signalling will generally be adequate on lines where traffic density is low. The required length of block section is much greater than braking distance. Only two types of signal are used, a stop signal showing stop and clear only and a distant signal showing caution or clear. Each stop signal will have its associated distant signal. As 2 aspect signalling will mainly be found outside the suburban area, the example shows single light signals. The distance (d) between stop signals is variable according to the geography of the line, positions of stations, loops etc. The headway distance can be calculated as: H = D + d + S + O + L giving a headway time:         Note that the headway time for the line is that of the longest section and cannot be averaged. To obtain the greatest signal spacing to achieve a specified headway, we transpose the equation to give: d = (V x T) - ( D + S + O + L)   2.3.  3 Aspect Signalling With 2 aspect signalling, as the required headway reduces, each stop signal will become closer to the distant signal ahead. it is therefore more economic to put both signals on the same post. This then becomes 3 aspect signalling. Each signal can display either stop, caution or clear. The distance (d) between signals must never be less than braking distance (D), but to ensure that the driver does not forget that he has passed a distant at caution, (d) should not be excessively greater than the service braking distance. The current SRA recommendation is for signal spacing to be no greater than three times braking distance. BR has adopted a maximum of 50% (i.e. 1.5D) although this is often exceeded at low speeds. The headway distance is given by:- H = 2d + S + O + L So the best possible headway, when the signals are as close as possible (exactly braking distance), is: H = 2D + S + O + L The headway with signals spaced 50% over service braking distance is: H = 3D + S + O + L The headway with signals spaced at three times braking distance is: H = 6D + S + O + L   2.4.  4 Aspect Signalling Where signals are closer together than braking distance, a preliminary caution or medium aspect is needed to give trains sufficient warning of a signal at danger. This medium aspect must not be less than braking distance (D) from the stop aspect, so the distance (d) between successive signals must on average be no less than 0.5D. The headway distance is given by:- H = 3d + S + O + L       where d > 0.5 D  So the best possible headway with 4 aspect signalling is given by:- H = 1.5 D + S + O + L In practice, the geographical constraints of the track layout will probably prevent regular spacing of signals at 0.5D. If the total length of two consecutive signal sections is less than braking distance, an additional medium aspect will be required at the previous signal. In other words, the first warning of a signal at stop must be greater than braking distance away. If more than two warnings are required, the medium aspect is repeated, not the caution. Signals should however be positioned so that this situation is as far as possible avoided. 2.5. Application of Low Speed Signals and Conditional Caution Aspects In normal use, the addition of a low speed signal provides the driver with a fifth aspect. It is important to realise that this does not have any effect on the headway of through or non-stopping trains running at their normal speed. In this situation, the engineer will arrange the signals so that each driver should, under normal conditions, see only clear aspects. The preceding headway calculations apply regardless of whether low speed signals are provided or not. A low speed signal tells the driver that he has little or no margin for error beyond the next signal and should control the speed of his train accordingly. The benefit of low speed signals is in allowing a second train to close up behind a stationary or slow moving train by reducing the length of the overlap, provided the speed of the second train has been sufficiently reduced. The same effect can be achieved by delaying the clearance of the caution aspect. This is now preferred, provided an overlap of the order of 100 metres can be achieved. The clearance of the signal should be delayed to give a passing speed of approximately 35km/h. Low speed signals should only be used where the reduced overlap is very short (less than 50 metres) and/or there are fouling moves within 100 metres of the stop signal. 2.5.1. Station Stops With an overlap of 500 metres, a train stopped at a station will have at least 500 metres of clear track behind it. A following train will stop at the first signal outside this distance. By the addition of a low speed signal or a conditionally cleared caution, the overlap distance can be reduced and the second train can approach closer to the station. When the first train leaves the station, the second train can enter the platform earlier, thus giving a better headway for stopping trains. A conditionally cleared caution aspect will normally be used unless the overlap is less than 50 metres. 2.5.2. Approaching Junctions Trains awaiting the clearance of another movement across a junction can approach closer to the junction while keeping the overlap clear of other routes across the junction. A low speed aspect will normally be used in this situation. 2.5.3. Recovery from Delays A line which is operating at or near its maximum capacity will be susceptible to disruption from even minor train delays (e.g. extended station stops at busy times). Low speed signals and or conditionally cleared caution aspects can allow trains to keep moving, even if only slowly, to improve recovery from the delay. The total length of a queue of trains will be less and the area over which the delay has an impact will be reduced. 2.6. Summary For 2 aspect signalling, the headway distance is:- H = D + d + [S + O + L]   For 3 aspect signalling, the headway distance is:- H = 2D + [S + O +  L]   (minimum) where signals are spaced at braking distance H = 2d + [S + O+ L] (general case) for an actual signal spacing of d   For 4 aspect signalling, the headway distance is:- H = 1.5 D + [S + O + L] (minimum) where signals are spaced at braking distance H = 3d + [S + O +  L]  (general case) for an actual signal spacing of d   Note the factor [S + O + L] is common to all equations.   Headway time is then calculated as:         2.7. Determining Signal Type and Spacing Because cost is generally proportional to the number of signals, the arrangement of signalling which needs the smallest number of signals is usually the most economic. It must, however, meet the headway requirements of the operators. For non-stop headways it is likely that the same type of signalling should be provided throughout. Otherwise there will be large variations in the headway. Remember that the headway of the line is limited by the signal section which individually has the greatest headway. This section will briefly describe a technique for determining the optimum signalling for a line. There may need to be localised variations (e.g. a 2 aspect signalled line may need 3 aspect signals in the vicinity of a station or a 3-aspect line may need to change to 4 aspect through a complex junction area). These variations will depend on the requirements for positioning individual signals and can be dealt with after the general rules have been determined. Firstly, determine braking distance, train length and overlap length required. Each must be the worst case. Knowing the required minimum headway, use the H = 2D + S + O + L equation to determine the best possible headway for 3 aspect. Compare the results with the required headway to check whether "best case" 3 aspect signalling is adequate. There should be a margin of 25–30% between the theoretical headway and that required by the timetable to allow for some flexibility to cope with delays. 2.7.1. If the Headway is Worse than Required 3 aspect will not be adequate and 4 aspect must be used. Recalculate for 4 aspect to confirm that this does meet the headway requirement. T = (1.5D + S + O + L) / V If the non-stop headway requires 4 aspect signalling, it is likely that station stops will cause further problems. Signal spacing near stations should be kept to a minimum and low speed signals or conditionally cleared cautions with reduced overlaps may also be required. 2.7.2. If the Headway is Much Better Much better generally means a headway time of 30% or less than that required by the timetable. If this is the case 2 aspect will generally be adequate. Calculate the greatest signal spacing that will achieve the headway with 2 aspect signalling. d=(V x T) - (D + S + O + L) Remember that in this distance d there will be two signals, a stop signal and a distant signal. Then compare this with the maximum permissible signal spacing for 3 aspect. In the absence of any firm rules, a judgement must be made on the amount of excess braking which is acceptable. SRA recommends that signal spacing is no more than three times braking distance while BR signalling principles specify no more than 1.5 times braking distance. If the two calculations produce a similar total number of signals (i.e. d for 2 aspect is approximately twice the value of d for 3 aspect) a 3 aspect system will be the better choice. The cost of the signals will be similar and the operator may as well benefit from the improved headway provided by 3 aspect. 2.7.3. If the Headway is Slightly Better It is probable that 3 aspect is the correct choice. Check that there is sufficient margin between the required and theoretical headway. 2.7.4. Signal Spacing Having evaluated that the chosen arrangement of signalling will provide the required headway, the relevant equation should be transposed to calculate the greatest possible signal spacing that can be allowed with the specified headway: eg. for 3 aspect signalling: V x T = H           = 2d + S + O + L therefore  2d     = (V x T) - (S + O + L) from which the post to post spacing (d) can be calculated Remember, there may be a constraint on the maximum signal spacing. The value of d should not exceed this. Geographical constraints may also require signals to be closer together than braking distance, in which case the 4th (medium) aspect is used where required. It does not need to be used throughout unless for headway [puposes]. 2.8. Example Information given:- Max. Line Speed...... 90 km/h Gradients ..........  Level Train Length.............. 250 metres Headway Required..... 2 1/2 mins. (non-stop) Before we start, we need the Service Braking Distance, either by calculation or from tables/curves (where available). We will assume that D = 625 metres. Note : S assumed to be 200 metres. O assumed to be 500 metres (although overlaps may need to be more accurately calculated if trainstops used) V = 90 km/h = 25m/s First, check 3 aspect signalling:- H = (2D + S + O + L) = (1250 + 200 + 500 + 250)  = 2200 metres so T = H/V = 88 seconds. This is much less than the 150 seconds (21/2 mins) specified. We will therefore consider the alternative of 2 aspect signalling. We cannot calculate a theoretical headway for 2 aspect signalling as the signal spacing is not fixed. Instead, we calculate the greatest 2 aspect signal spacing to give us the 150 second headway specified : d = (V x T) - (D + S + O + L) = (25 x 150) - (625 + 200 + 500 + 250) = 3750 - 1575 metres = 2175 metres Hence 2 aspect signalling, with the stop signals no more than 2175 metres apart, would give the 2 1/2 min. headway required. However, each stop signal also requires a distant signal. Two signals are therefore required every 2175 metres. 3 aspect signalling with signals every 1088 metres would require no more signals but would give a better headway of: H = 2d + (S + O = L) = 2175 + (200 + 500 + 250) = 3125 metres So T = H/V = 3125/25 = 125 seconds In fact, the signal spacing could be extended further within the headway requirement of 150 seconds. This would give a better headway with fewer signals than 2 aspect. This demonstrates that 2 aspect is generally worth considering only for very long headways. We could now calculate the maximum possible 3 aspect signal spacing allowed by the headway : V x T = H = 2d + S + O + L therefore 2d      = (V  x  T) - (S + O + L) = (25 x 150) - (200 + 500 + 250) = 3750 - 950 = 2800 metres d        = 1400m As this is over twice braking distance, it should be confirmed that this signal spacing is operationally acceptable TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 2...........

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